This proposal focuses on deciphering myosin II isoform roles and regulation during cytokinesis and during polarized cell migration and chemotactic responses. Although myosin II is a critical and ubiquitous force producing enzyme, its exact mechanical roles during cellular contractile events is poorly understood, and there is virtually no understanding of isoform specific roles. Multiple conflicting models exist regarding how myosin II filament assembly and localization is regulated in vivo, and none of the prevailing models have been tested in vivo. We will use in vivo imaging of GFP-myosin II reporter proteins, fluorescent recovery after photobleaching (FRAP) technology, as well as siRNA and other approaches to generate significant new understanding of fundamental mechanisms of myosin II regulation and new understanding of the isoform-specific roles of myosin MA and MB in processes such as cytokinesis and polarized cell migration. Our studies will also identify the key upstream protein kinases and regulatory proteins responsible for activation of myosin II assembly and contractility during cytokinesis.